[PATCH v3 7/8] mm: drop VMA lock before waiting for migration

[Suren Baghdasaryan <surenb@google.com>]

migration_entry_wait does not need VMA lock, therefore it can be
dropped before waiting.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/memory.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 5caaa4c66ea2..bdf46fdc58d6 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3715,8 +3715,18 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	entry = pte_to_swp_entry(vmf->orig_pte);
 	if (unlikely(non_swap_entry(entry))) {
 		if (is_migration_entry(entry)) {
-			migration_entry_wait(vma->vm_mm, vmf->pmd,
-					     vmf->address);
+			/* Save mm in case VMA lock is dropped */
+			struct mm_struct *mm = vma->vm_mm;
+
+			if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
+				/*
+				 * No need to hold VMA lock for migration.
+				 * WARNING: vma can't be used after this!
+				 */
+				vma_end_read(vma);
+				ret |= VM_FAULT_COMPLETED;
+			}
+			migration_entry_wait(mm, vmf->pmd, vmf->address);
 		} else if (is_device_exclusive_entry(entry)) {
 			vmf->page = pfn_swap_entry_to_page(entry);
 			ret = remove_device_exclusive_entry(vmf);
-- 
2.41.0.178.g377b9f9a00-goog